This invention relates to an objective lens for condensing a light beam, and particularly to an objective lens which is advantageously used in an optical pickup device and an optical pickup device using this objective lens.
Conventionally, an optical pickup device has been proposed which enables recording or reproduction of information signals with respect to a plurality of optical recording media of different standards, such as different information signal recording densities and different recording capacities, for example, an optical disc of the DVD (digital versatile disc) standard and an optical disc of the CD (compact disc) standard.
An optical pickup device of this type is described in the Japanese Publication of Unexamined Patent Application No.H7-98431. In the optical pickup device described in this publication, a hologram is formed at a part of the surface of an objective lens so as to correct the spherical aberration, which is a problem in the case of using optical discs of different standards. The objective lens used in this optical pickup device is formed to condense a light beam with a wavelength of 650 nm onto the signal recording layer of the optical disc at a numerical aperture (NA) of 0.6 via a cover layer with a thickness of 0.6 mm, for example, the disc substrate of the optical disc. The hologram formed on the surface of the objective lens is formed to condense a first-order light of a light beam with a wavelength of 780 nm onto the signal recording layer of the optical disc at a numerical aperture (NA) of 0.45 via a cover layer with a thickness of 1.2 mm, for example, the disc substrate of the optical disc.
The optical pickup device carries out recording and reproduction of information signals to and from an optical disc of the DVD standard by using a zeroth-order light transmitted through the hologram, of a light beam with a wavelength xcex of 650 nm, and a transmitted light in the area where the hologram is not formed. The optical pickup device also carried out recording and reproduction of information signals to and from an optical disc of the CD standard by using a first-order light diffracted by the hologram, of a light beam with a wavelength xcex of 780 nm.
In the optical pickup device having the objective lens as described above, the diffraction efficiency of the zeroth-order light transmitted through the hologram, of the light beam with a wavelength xcex of 650 nm, and the diffraction efficiency of the first-order light in the hologram, of the light beam with a wavelength xcex of 780 nm, cannot be increased to approximately 40% or more. In this optical pickup device, the use efficiency of a back-and-forth light beam, which is emitted from the light source, passed through the objective lens and a diffractive optical element to reach the optical recording medium, then reflected from the optical recording medium, again passed through the objective lens and the diffractive optical element, and received by a light-receiving element, is 16% or lower since the beam passes through the diffractive optical element twice.
The optical pickup device using the hologram also has a problem of a stray light due to an unwanted diffracted light.
Moreover, in the case where the objective lens is made of a synthetic resin, there is a problem of aberration, generated by a change in the refractive index of the synthetic resin due to a temperature change.
In view of the foregoing status of the art, it is an object of the present invention to provide a new optical pickup device and a new optical lens which overcome the problems of the conventional optical pickup device and objective lens.
It is another object of the present invention to provide an optical pickup device and an objective lens which enable recording and reproduction of information signals to and from a plurality of types of optical recording media having different information signal recording densities, while efficiently using a light beam emitted from a light source.
It is still another object of the present invention to provide an optical pickup device and an objective lens which enable restraint of generation of a stray light due to an unwanted diffracted light and generation of aberration due to a temperature change.
An objective lens according to the present invention comprises a convex lens and a diffractive optical element provided at least on one surface or in the vicinity of the convex lens. When the convex lens condenses an incident light beam onto a condensation surface via a parallel plate with a predetermined thickness, correction of the spherical aberration is insufficient in the state where the diffractive optical element does not exist, and a light beam diffracted by the diffractive optical element is condensed onto the condensation surface in the state where the spherical aberration is corrected.
An optical pickup device according to the present invention comprises two light sources for emitting light beams with different wavelengths, an objective lens for condensing the respective light beams emitted from the respective light sources onto a signal recording surface of an optical recording medium via a transparent cover layer provided on the optical recording medium, a light beam branch element for branching a light beam reflected by the signal recording surface from an optical path returning to the light source, and a photodetector for receiving the light beam branched by the light beam branch element. The objective lens is made up of a convex lens and a diffractive optical element provided at least on one surface or in the vicinity of the convex lens, and condenses diffracted lights of the same order generated by the diffractive optical element, of the two light beams with different wavelengths emitted from the respective light sources, onto the signal recording surface, thereby condensing the respective light beams onto the signal recording surface in the state where the spherical aberration generated in the transparent cover layer is corrected.
An optical pickup device according to the present invention comprises a light source for emitting a light beam, an objective lens for condensing the light beam emitted from the light source onto a signal recording surface of an optical recording medium, a light beam branch element for branching a light beam reflected by the signal recording surface from an optical path returning to the light source, and a photodetector for receiving the light beam branched by the light beam branch element. The objective lens used in this device is made up of a convex lens made of a synthetic resin and a blazed-shape hologram provided at least on one surface of the convex lens, and uses a plus-first-order diffracted light or a minus-first-order diffracted light of the hologram, thereby canceling the spherical aberration generated by a change in the refractive index of the convex lens made of the synthetic resin due to a temperature change.
Other objects and specific advantages of the present invention will be clarified further from the following description of embodiments.